The airspeed of helicopters is inter alia limited by aerodynamic effects at the rotor blades. Due to the superposition of rotational speed and air speed, high flow velocities are present on the advancing side of the horizontal rotor which if too high can result in local shock waves at the advancing rotor blades. On the retreating side of the horizontal rotor, the flow velocity at the rotor blades with regard to the surrounding air is reduced. In an inner area close to the rotor axis, the flow velocity may even be reversed, resulting in a flow from the trailing edge to the leading edge of the rotor blade. For the purpose of providing equal lift distribution or ascending forces on both sides of the horizontal rotor, despite the strongly differing rotor blades speeds with regard to the surrounding air, the incidence or angle of attack of the rotor blades has to be increased on the retreating side and decreased on the advancing side of the horizontal rotor. Thus, the incidence of the rotor blades has to be periodically varied over each rotation of the horizontal rotor of a helicopter flying forward. These variations result in varying angle of attack and may locally cause a dynamic stall. The dynamic stall causes a short increase in lift followed by a sharp reduction or loss of the lift. As a result, there are strong variations in the drag and in the pitch moment of the rotor blade. This high frequency transient aerodynamic effect results in high structural loads on the rotor blades and the entire drive and control system of a helicopter, which is accompanied by a strong excitation of vibrations. A further increase of the air speed of a helicopter is not possible without taking measures to unload its horizontal rotor.
In a rotor blade for a helicopter known from DE 198 08 196 C2, a movable flap is provided at the trailing edge of the rotor blade. The angle of the flap is actively adjustable with regard to the remainder of the rotor blade. The flap is connected to the main body of the rotor blade via a flexible fiber joint. By means of the flap of this known rotor blade, it is intended to increase the rotor performance and to simultaneously reduce the vibration level. The connection of the flap to the main body via the flexible fiber joint is intended to reduce the wear in the area of the flap as compared to flaps which are connected via common two-part pivot joints. The known rotor blade having an actively operated flap is very complex as compared to a rotor blade without flap.
A rotor blade with advanced behavior with regard to stall occurring at strongly varying angles of attack is known from DE 10 2005 018 427 A1. Here, devices are arranged in the area of the profile front edge (leading edge) to enhance the behavior at increased angle of attack. These devices include at least one passive vortex generator which is at or close to the stagnation point of the rotor blade with low angles of attack so that it is out of function, and which emerges out of the boundary layer of an accelerated flow on the underpressure side of the rotor blade with increased stall angles so that it starts to operate at higher angles of attack.
Up to now, there is no system ready for serial production for controlling the dynamic stall at rotor blades of helicopters. The selection of the blade profile and the profile distribution in radial direction have an influence. In this selection, however, other boundary conditions are to be met. For example, a wide area of Mach and Reynolds numbers and of angle of attack have to be covered. In practice, the area in which dynamic stall occurs is up to now avoided by limiting the overall air speed.
Experimental research in a wind tunnel has been carried out with regard to surface flaps on airfoils of fixed wing airplanes to evaluate the potential of these devices to suppress the reversed flow that can occur locally on the airfoils when the flow separates at high angles of attack (see Meyer, Robert K. J., “Experimentelle Untersuchungen von Rückstrombremsen auf Tragflügeln zur Beeinflussung von Strömungsablösungen”, Disseration TU Berlin Hermann-Föttinger-Institut für Strömungsmechanik, Mensch & Buch Verlag, ISBN 3-89820-205-4).
DE 198 59 041 C1 discloses an adjustable blade profile for a rotor blade of a rotary wing airplane. The blade profile is adjustable about an axis running in parallel to the radial axis of the rotor blade. This adjustment of the entire blade profile shall take place automatically due to variations of the lift operating against an elastic means under the influence of the centrifugal force on the rotor blade. This concept, however, has not been applied in practice.
The conditions at a rotor blade of a helicopter basically differ from the conditions at an air foil of a fixed wing airplane. The particularity of a helicopter is the blade velocity with regard to the surrounding air, which strongly varies periodically during one rotor revolution. The stall at a rotor blade of a helicopter thus takes place within much shorter time, and generally the stall emerges from the leading edge of the blade profile. At fixed-wing airfoils, the stall mostly begins at the trailing edge.
A four-bladed rotor of a helicopter of the 2,000 kg-class (such as the helicopter EC 135), for example, rotates at a frequency of 6.5 Hz corresponding to an angular speed of 41 rad/sec. The dynamic stall due to the variation of the angle of attack of the rotor blade over each rotation takes place in an azimuthal area of about 60°, which equals approximately 1 rad. This area is passed by the rotor blade within approximately 0.02 sec.
Considering a helicopter, the dynamic stall occurs at high forward flight speeds and limits the maximum flight speed of the helicopter. For a fixed wing airplane, stall occurs at too low air speeds and limits the minimum air speed.
There still is a need for a rotor blade in which the negative effects associated with dynamic stall, such as loss of lift, increase of drag and pitching moment and thus increased vibrations and loads and component stress, is shifted towards higher angles of attack and thus to higher forward flight velocities by means of simple devices not affecting the integrity of the main body of the rotor blade.